The Cosmic Barcode

Planets, giant gas clouds, stars, supernova explosions, the
violent core of galaxies where possible black holes reside, all emit radiation in
very specific regions of the electromagnetic spectrum.
Their characteristic emissions, called spectral lines,
contain a wealth of information about these objects. These "cosmic barcodes" have
revealed much of what we know about the cosmos.

Spectral line emissions can fall within any region of the spectrum, including the radio
region, the visible, and the X-ray region, and indicate the composition and structure
of their source. While spectral lines are identifiable because they are expected to
have very specific frequencies, their frequency often appears to be shifted by the Doppler Effect. Doppler shifts result from the motions, relative to the observer, of the objects emitting the radiation.